1. Field of the Invention
The present invention relates to a method for thermoforming a resilient, fluid-filled bladder for use in a variety of applications, including footwear soles.
2. Description of Background Art
Footwear is divided into two general parts, an upper and a sole. The upper is designed to comfortably enclose the foot and the sole provides traction, protection, and a durable wear surface. The considerable forces generated in athletics require that the sole of footwear provide enhanced protection and shock absorption for the foot and leg. Accordingly, the sole of athletic footwear typically includes several layers, including a resilient, shock-absorbent midsole and a ground-contacting outsole which provides both durability and traction.
Such midsoles have been formed of conventional foam materials, for example, ethylene vinyl acetate or polyurethane which compress resiliently under an applied load and provide cushioning. Outsoles have been made of conventional wear-resistant materials such as a carbon-black rubber compound. Conventional foam materials are resiliently compressible, in part, due to the inclusion in the foam of open or closed cells defining an inner volume that is substantially displaced by gas. That is, the foam can include bubbles formed in the material which include air therein. However, after repeated compressions, the cell structure collapses, resulting in decreased compressibility of the foam. Thus, the overall cushioning of the midsole deteriorates.
One way to overcome the drawbacks of using conventional foam materials is disclosed in U.S. Pat. No. 4,183,156 to Marion F. Rudy, hereby incorporated by reference, in which cushioning is provided by inflatable inserts made of elastomeric materials. The inserts include a plurality of tubular chambers which extend substantially longitudinally throughout the length of the footwear. The chambers are in fluid communication with each other and jointly extend across the width of the footwear. In U.S. Pat. No. 4,219,945 to Marion F. Rudy, hereby incorporated by reference, an inflated insert is encapsulated in a foam material. The combination of the insert and the encapsulating material functions as the midsole. An upper is cemented to the upper surface of the encapsulating material and an outsole or tread member may be fixed to the lower surface.
U.S. Pat. No. 4,817,304 to Mark G. Parker, et al., hereby incorporated by reference, discloses a foam-encapsulated, gas-filled insert in which gaps are left along the sides of the encapsulating member. When the midsole is compressed, the insert expands into the gaps. Thus, the gaps provide decreased stiffness in compression of the midsole, while reducing the overall weight of the footwear. Further, by appropriately locating the gaps, the overall impact response characteristics can be tuned along areas of the footwear.
Such bladders generally are made of an elastomeric material and are formed so as to have an upper or lower surface enclosing one or more chambers therebetween. The chambers are pressurized above ambient pressure by insertion of a nozzle or needle connected to a fluid pressure source into a fill inlet formed in the bladder. After the chambers are pressurized, the fill inlet is sealed, for example, by welding, and the nozzle is removed.
Bladders of this type have been manufactured by the prior art two-film technique in which two separate sheets of elastomeric film are formed having the overall peripheral shape of the bladder. The sheets are welded together along the periphery to form a bladder having upper, lower, and side surfaces, and at predetermined interior areas to give the bladder a desired configuration; that is, to have chambers of a predetermined shape and size at desired locations.
Bladders have also been manufactured by the prior art blow-molding technique, wherein a liquefied elastomeric material is placed in a mold having the desired overall shape and configuration of the bladder. The mold has an opening at one location through which pressurized air is provided. The pressurized air forces the liquefied elastomeric material against the inner surfaces of the mold and causes the material to harden in the mold to form a bladder having the desired shape and configuration.
Another type of prior art bladder used in soles of footwear is disclosed in U.S. Pat. Nos. 4,906,502 and 5,083,361, both to Marion F. Rudy, and both hereby incorporated by reference. The bladders disclosed in the '502 and '361 patents are formed as a gas pressurized and inflated structure which comprises an hermetically sealed outer covering barrier layer which is securely bonded substantially over the entire outer surfaces of a double-walled fabric core. The double-walled fabric core is comprised of first and second outer fabric layers which are normally spaced apart from one another at a predetermined distance. Connecting or drop yarns, potentially in the form of multi-filament yarns comprised of many individual fibers, extend internally between the proximal or facing surfaces of the respective fabric layers. The filaments of the drop yarns form tensile restraining means and are anchored to the respective fabric layers. A suitable method of manufacturing the double walled fabric structure is double needle bar Raschel knitting.
U.S. Pat. Nos. 5,993,585 and 6,119,371, both issued to David A. Goodwin et al., and both hereby incorporated by reference, disclose a bladder utilizing a double-walled fabric core, as with the '502 and '361 patents, but without a peripheral seam located midway between the upper and lower surfaces of the bladder. Instead, the seam is located adjacent to the upper surface of the bladder. Advantages in this design include removal of the seam from the area of maximum sidewall flexing and increased visibility of the interior of the bladder, including the connecting yams. The process used to form a bladder of this type, according to the '585 patent, involves the formation of a shell, which includes a lower surface and a sidewall, with a mold. A double-walled fabric core is placed on top of a covering sheet, and the shell, following removal from the mold, is placed over the covering sheet and core. The assembled shell, covering sheet, and core are then moved to a lamination station where radio frequency energy bonds opposite sides of the core to the shell and covering sheet and bonds a periphery of the shell to the covering sheet. The bladder is then pressurized by inserting a fluid so as to place the connecting yarns in tension.
A process for thermoforming a bladder is disclosed in U.S. Pat. No. 5,976,451 to Joseph J. Skaja et al., hereby incorporated by reference, wherein a pair of flexible thermoplastic resin sheets are heated and placed against a pair of molds, a vacuum drawing the sheets into the mold. The sheets are then pressed together to form a bladder.